This invention relates to devices for measuring the concentration of contaminants or foreign materials in a fluid and, more particularly, to nephelometers for measuring the concentration of particulate materials in liquids.
A nephelometer generally is an optical instrument for measuring the concentration of particular contamination in a fluid, usually air. It does so by measuring the intensity of light scattered by clouds of particles in a known volume of the fluid being measured. Turbidity meters are commonly used to measure the quality of liquids, particularly water. They do so by measuring the attenuation of transmitted light through a solution containing finely divided particles in suspension. There is a direct relationship between the amount of light attenuated and the amount of material in suspension.
A nephelometer normally is calibrated in micrograms per cubic meter with a minimum response in the order of one microgram per cubic meter. A turbidity meter is calibrated in Nephelometric Turbidity Units (NTU). A nephelometer has greater sensitivity, accuracy and provision in measuring small amounts of turbidity. For example, the minimum response of higher grade turbidity meters is in the order of 0.01 NTU which is equivalent to approximately 30 milligrams per cubic meter. The sensitivity of both type instruments is limited by the light scatter collection angle (defined by the numerical aperture of the optical collection system), by the size of the viewing volume (defined by the optical depth of focus) and by the background stray light level inherent in the sensor.